evolution [part 3]
TRANSCRIPT
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EVOLUTION
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TOPIC OUTLINE
A)The pattern of evolutionB) Evolutionary agents
C) Artificial selection
D)Polymorphism
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Biological Evolution:
is a change over time in the genetic composition of a population
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Evolution creates two patterns across:
1. TIME:
vertical evolution – a change in a single lineage through time
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Evolution creates two patterns across:
2. SPACE:
speciation (divergent evolution) - the process by which one evolutionary unit splits into two units, which thereafter evolve as distinct lineages
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Darwin in his book ‘On the Origin of Species’:
was concerned with vertical evolution
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Darwin in his book ‘On the Origin of Species’:
did not discuss how a single species splits into two or more daughter species
Darwin in 1840
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Darwin:
proposed a mechanism (natural selection) to explain adaptation
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Adaptation has two meanings in evolutionary biology:
a characteristic (anatomy, physiology, behaviour) of an organism that is well suited to its environment
the process by which a population becomes better suited to its environment (better adapted)
adaptation is not perfect, just an improvement on what came before
Or
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Microevolution vs Macroevolution
Microevolution or adaptive evolution:
refers to changes in allele frequencies
plant and animal breeding are examples of microevolution
Macroevolution or speciation :
is an evolutionary process by which species originate
the fossil record provides the best evidence
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Question: [MAY, 2010]
Natural selection is the only evolutionary process that leads to adaptation.
What is an adaptation? (1)
Adaptations are structures or behaviours that allow efficient use of the environment. For example, the webbed foot of a duck enables it to swim better than a foot that is not webbed. Adaptations are due to genes, that is, they are inherited.
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Two opposing views of evolution models:
1. Gradualism 2. Punctuated equilibrium
Time
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What does ‘Punctuated equilibrium’ mean?
Long periods of stasis (equilibrium) are broken up (punctuated) by bursts of
speciation.
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Both rely on the same evidence: fossils
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They differ in how they:
interpret gaps in the fossil record
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Extreme views of gradualism vs. punctuated equilibrium
Gradualism Punctuated equilibrium
Mutations & phenotypical changes are gradual and explain the fossil record gaps as simply missing because fossils are hard to find.
In long periods of time, mutations simply accumulate but do not cause any drastic phenotypical changes, followed by short periods where these mutations are suddenly expressed. This would account for the lack of transitional fossils.
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Interpreting an evolutionary tree
a graph of time against change in form
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Question: [SEP, 2001]
How may the following observation be interpreted in the light of evolutionary theory?
Viral DNA has the same basic structure as the DNA of prokaryotes and eukaryotes.(2)
All organisms originated from a common ancestor. Thus DNA of all organisms has the same basic structure.
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Queston: [MAY, 2009]
The graph in Figure 1 shows the change in the length of a vertebrate limb (y-axis) over geological time (x-axis). The labels A and B on the graph represent time intervals. During the period being considered, the limb changes from length β1 to length β2, as shown in the graph. A palaeontologist studying this species concludes that the change from β1 to β2 is sufficient to make this a speciation event.
Figure 1: Change in the length of a vertebrate limb over geological time
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1. What is speciation? (2)
Speciation is the evolutionary process by which new biological species arise.
2.During which time interval, A or B, is speciation occurring? Give a reason for your answer. (2)
B - the change in the length of a vertebrate limb is taking place.
Figure 1: Change in the length of a vertebrate limb over geological time
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3. What model of evolutionary change does this speciation event seem to be following? (1)
Punctuated equilibrium.
Figure 1: Change in the length of a vertebrate limb over geological time
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4 Briefly describe the principal characteristics of this model of evolutionary change. (3)
Characteristics of a species remain unchanged for a long period of time, then in a relatively short span, changes occur which lead to new species in such a way that no intermediate forms exist. This accounts for gaps in the fossil record.
5.Name ONE other model of evolutionary change and briefly describe its basic principles. (3)
Gradualism model. Evolutionary change occurs very slowly and at a constant rate. Intermediate forms thus are expected.
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TOPIC OUTLINE
A) The pattern of evolution
B)Evolutionary agentsC) Artificial selection
D)Polymorphism
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What are ‘Evolutionary agents’?
1. mutation
2. gene flow
3. random genetic drift
4. non-random mating
5. natural selection
Forces that change allele and genotypic frequencies in populations
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Evolutionary agents cause deviations from the
Hardy-Weinberg equilibrium
Because such changes in the gene pool constitute small-scale evolutionary changes, they are referred to as microevolution m
acro
evo
luti
on
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1. Mutations
mutation rates are generally so low that they have little effect on the Hardy-Weinberg proportions of common alleles
typical mutation rate is 1 in a million per generation
Exposure to chemicals or
radiation
DNA fails to copy accurately
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1. Mutations
mutations can restore to populations alleles that other evolutionary agents remove
mutations both: create and help maintain variation within
populations
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2. Gene flow
gene flow happens when migrating individuals breed in their new location
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Immigrants may:
add new alleles to the gene pool of a population
change the frequencies of alleles already present if they come from a population with a different allele frequency
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3. Genetic drift
is a change in the pool of genes of a small population that takes place strictly by chance
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Explain why although genetic drift is a mechanism of evolution, it does not work to
produce adaptations.
It is an entirely RANDOM process.
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Genetic drift can result in genetic traits :
being lost from a population
becoming widespread in a population without respect to the survival or reproductive value of the gene pairs (alleles) involved
OR
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Effects of genetic drift on a small population are large:
the marble-drawing scenario illustrates why drift affects small populations more
bag has only 20 marbles (a tiny bag!) & you can only draw four marbles to represent gene frequencies in the next generation
something like this might happen:
Notice how quickly and drastically the marble ratio changed: 1:1, 1:3, 0:1.
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What is the ‘bottleneck effect’?
When the size of a population is drastically reduced due to an event/disaster
E.g. during the 1890’s hunting reduced the number of elephant seals to about 20 in a single population
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During population bottlenecks, genetic variation can be reduced by
genetic drift
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The ‘founder effect’:
is a form of bottlenecking
a small group in a population splinters off from the original population and forms a new one
Island
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The founder effect is a special case of genetic drift
by chance alone, a few individuals that establish a new population have allele frequencies that differ from those of the original population
Original population
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4. Non random mating occurs when individuals choose mates
with particular phenotypes
The Zoo’s Panda breeding program was still getting nowhere.
TOO FAT, TOO WHITE, TOO SMALL….
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Mating patterns may alter genotype frequencies
if individuals in a population do not choose mates at random
E.g., if they mate preferentially with individuals of the
same genotype, then:
AA x AAaa x aaAa x Aa
100% AA 100% aa25% AA : 25% aa: 50% Aa
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Homozygous genotypes will be overrepresented
Heterozygous genotypes will be underrepresented relative to
the Hardy-Weinberg expectations
100% AA 100% aa25% AA : 25% aa: 50% Aa
AA x AAaa x aaAa x Aa
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Alternatively, individuals may mate
primarily or exclusively with individuals of different genotypes
nonrandom mating is seen in some plant species, such as primroses where pin-eyed flowers mate with thrum-eyed flowers
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Self fertilisation (selfing): another form of nonrandom mating
is common in many organisms, especially plants
Selfing:
1. reduces the frequencies of heterozygous individuals from Hardy-Weinberg equilibrium
2. increases the frequencies of homozygotes
3. does not change the allele frequencies
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5. Natural Selection
What is ‘Selection’?
The process by which those organisms which appear physically, physiologically and behaviourally better adapted to the environment survive and reproduce; those organisms not so well adapted either fail to reproduce or die
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Two types of selection:
1. Artificial selection:
a breeder selects for the desired characteristics
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2. Natural selection:environmental conditions determine which individuals in a population produce the most offspring
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Generation 1: 1.00 not resistant0.00 resistant
Resistance to antibacterial soap
How natural selection works
Resistance to antibacterial soap
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Resistance to antibacterial soap
Generation 1: 1.00 not resistant0.00 resistant
How natural selection works
Resistance to antibacterial soap
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Resistance to antibacterial soap
mutation!
Generation 1: 1.00 not resistant0.00 resistant
Generation 2: 0.96 not resistant0.04 resistant
How natural selection works
Resistance to antibacterial soap
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Resistance to antibacterial soap
Generation 1: 1.00 not resistant0.00 resistant
Generation 2: 0.96 not resistant0.04 resistant
Generation 3: 0.76 not resistant0.24 resistant
How natural selection works
Resistance to antibacterial soap
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Resistance to antibacterial soap
Generation 1: 1.00 not resistant0.00 resistant
Generation 2: 0.96 not resistant0.04 resistant
Generation 3: 0.76 not resistant0.24 resistant
Generation 4: 0.12 not resistant0.88 resistant
How natural selection works
Resistance to antibacterial soap
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Selection determines which alleles are passed to the next generation
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natural selection (the process) can lead to evolution (the outcome), but natural selection is only one of several processes that can result in evolutionary change
NATURAL SELECTION
Mutations cause variation in traits
Bad
Die
No offspring
Good = adaptation
Survive
Reproduce and pass adaptation to
offspringEVOLUTION
predators
shelterenvironmental
factors
amount of food
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moreover, natural selection can occur without producing evolutionary change;
only if variation is genetically based will natural selection lead to evolution
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Selection pressure is a means of:
increasing
or decreasing
the spread of an allele within the gene pool
What can these changes in allele frequency lead to?
Evolutionary change
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Three conditions for natural selection to occur and to result in evolutionary change:
1. Variation must exist among individuals in a population.
3. Variation must be genetically inherited.
2. Variation among individuals must result in differences in the number of offspring surviving in the next generation.
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Selection processes
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competition sets in as certain factors become
limiting e.g. food
What happens when a population increases in size?
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Which organisms survive and reproduce?
those showing characteristics which give them a competitive advantage to obtain the resources
organisms without, die before reproducing
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Environmental limiting factors
&
Population size
What is a ‘selection pressure’?
A selection pressure is an abstract force that shapes organisms as
they evolve
operate together to produce a selection
pressure
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Potential sources of selection pressure:
Availability of preyPresence of predatorsObtaining foodCompetition with other speciesParasitesCompetition for a mateClimate
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Potential sources of selection pressure:
Availability of prey variants with adaptations allowing them
to obtain food have more offspring
- e.g. Speed, senses for finding prey/food, weapons for killing prey
or obtaining food, camouflage for stealth
Sole
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Potential sources of selection pressure:
Presence of predators
variants with adaptations allowing them to escape predators have more offspring
- e.g. speed, defensive weapons, camouflage, mimicry
A noxious mixture shoots into the face of an attacker with a loud pop at boiling temperatures.
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- camouflage
Can you spot the frog?
- mimicry
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Potential sources of selection pressure:
The leaf bug
The selection pressure is predators
Its strategy is to mimic a leaf
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Question: [SEP, 2004]
Comment on the biological significance of the following observation: Removal of predators may reduce
diversity of prey organisms. (2)
Predators create a selection pressure, favouring certain phenotypes instead of others. Removal of this pressure, no longer drives populations to diversify.
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Potential sources of selection pressure:
Competition for a mate variants with adaptations allowing them
to attract a mate to have offspring
- e.g. strong, attractive, good provider
Zebra finches.
Brightness of male’s bill is an indicator of health.
Color is due to carotenoids, which are also important in the immune system. A brighter bill indicates more carotenoids and greater overall health.
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Original population
(c) Stabilising selection(b) Disruptive selection(a) Directional selection
Phenotypes (fur color)
Fre
qu
ency
of
ind
ivid
ual
s
Originalpopulation Evolved
population
Three types of selection processes:
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1. Stabilising Selection
Natural selection favors the average for population selected
Happens when the extremes of the trait are not as well suited
E.g. tail length of birds
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Example: Stabilising Selection
Human birth weight underweight or overweight babies die,
therefore a certain weight is favoured
a study on 13, 730 babies [1935 -1946] showed that there is an optimum birth weight of about 3.6 kg
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2. Directional Selection
Natural selection favors one extreme of the population for that trait
often happens when environment changes in a consistent way- e.g. climate gets colder
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Examples: Directional Selection
Peppered moths before and after the industrial revolution
Up to 1848:
All moths were creamy white
In 1848:
A black form was reported
By 1895:
19% were black
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Examples: Directional Selection
Insecticide resistance
Random mutation
Resistant insects survive
Resistant insects multiply
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Examples: Directional Selection
Selective breeding
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3. Disruptive Selection
Natural selection favors both extremes selected
Causes species to diverge
[polymorphism]
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Disruptive Selection Results in a Bimodal Distribution
bill size in black-bellied seed crackers
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Tim
eStabilising selection
Disruptive selection
Directional selection
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What is ‘fitness’?
Fitness describes how good a particular genotype is at leaving offspring in the next generation relative to how good
other genotypes are at it
Lower fitness Higher fitness
F1
F2
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I’m proud to
be so fit !
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A genotype’s fitness includes:its ability to survivefind a mateproduce offspring—and
ultimately leave its genes in the next generation
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These strategies help the parents get more of their offspring into the next generation.
Strategies which increase fitness:
Parental care
Producing thousands of young
Sporting fancy feathers that attract females
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TOPIC OUTLINE
A) The pattern of evolution
B) Evolutionary agents
C)Artificial selectionD)Polymorphism
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Artificial Selection :
is the gradual improvement of animal and plant characteristics over time, for human benefit
the term was utilised by Darwin in contrast to natural selection
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Over thousands of years man has selected characteristics in dogs that are useful…..
•Very small dogs•Used for pets
•Large hunting dogs•Strong & powerful
•Large working dogs•Strong and athletic
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Look at the variety of plants that have been artificially selected from mustard
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What is ‘inbreeding’?
Mating of closely related individuals
Twins – result of inbreeding
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Advantages of inbreeding:
For example:
Parental genotypes: Ffgg x Ffgg
Gametes: Fg Fg
F1 genotypes: FFgg
(pure breeding)
X
Create pure line over time
Better adapted to steady environment
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Disadvantages of Inbreeding:
1. Increasing the chance for recessive genes to be homozygous
2. Most recessive genes are undesirable traits
3. Leads to a high frequency of defects present at birth
4. Reduce the genetic variability vigour and fertility of a population.
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How do breeders overcome the problems of inbreeding?
Breeders resort to outbreeding after
several generations of inbreeding
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What is ‘Outbreeding’?
crossing individuals from genetically distinct populations
outbreeding usually takes place between: members of different varieties
or strains
in certain plants between closely related species
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Advantages of outbreeding:
1. Progeny are heterozygous and the bad recessive genes are masked by normal dominant alleles.
2. Hybrid vigour – Progeny are tougher, more fertile and have a greater chance of survival.
3. Produces variation/heterozygosity; on which natural selection can act.
No more pure line exists
Disadvantage:
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A simple genetic explanation of increased vigour in F1 hybrids
Parental genotypes (2n) :
FFgghhIIjj x FFGGHHiiJJ
Gametes (n) :
F1 genotype (2n) : FFGgHhIiJj
FghIj
FGHiJ
X
Hybrids have characteristics superior to either of the parental stocks: the
phenomenon is called
hybrid vigour
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TOPIC OUTLINE
A) The pattern of evolution
B) Evolutionary agents
C) Artificial selection
D)Polymorphism
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Polymorphism is the:
existence of two or more forms of the same species within the same population, and can apply to:
biochemical morphological behavioural
characteristics
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Polymorphism plays a significant role in the process of natural selection. Why?
As different phenotypes are produced
Shell color polymorphism in the snail, Liguus fascitus
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Two forms of Polymorphism:
1)transient polymorphism
2) balanced or stable polymorphism
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Two forms of Polymorphism:
1) transient polymorphism arises when different forms (morphs),
exist in a population undergoing strong selection pressure
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Transient Polymorphism:
usually applies in situations where one form is gradually being replaced by another
e.g. the peppered moth
Normal situation:
peppered (grey) form more numerous than melanic (black) form
Industrial Revolution:
peppered (grey) form less numerous than melanic (black) form
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Two forms of Polymorphism:
2) balanced or stable polymorphism occurs when different forms coexist
in the same population in a stable environment
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Examples of Balanced Polymorphism:
1. the existence of the two sexes in animals and plants
The sex ratio for the entire world population – 101 males :100 females
2. in humans, the existence of the A, B, AB and O blood groups
3. red-green colour blindness in humans
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Examples of Balanced Polymorphism:
1. the existence of the two sexes in animals and plants
The sex ratio for the entire world population – 101 males :100 females
2. in humans, the existence of the A, B, AB and O blood groups
3. red-green colour blindness in humans
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Examples of Balanced Polymorphism:
4. the existence of workers, drones and queens in social insects 5. pin-eyed and thrum-
eyed forms in primroses
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0–2.5%
Distribution ofmalaria caused byPlasmodium falciparum(a parasitic unicellular eukaryote)
Frequencies of thesickle-cell allele
2.5–5.0%
7.5–10.0%
5.0–7.5%
>12.5%
10.0–12.5%
6. sickle-cell anaemia
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Question: [MAY, 2007]
Use your knowledge of biology to explain the following statement.
Islands generally comprise several endemic species.
(5 marks)An endemic species is found only on one island. This is the result of allopatric speciation. No gene flow between populations on different islands – sea is a geographical barrier. Different mutations may occur on the different islands. Islands could have different selection pressures in the form of climate, predators and food sources. Chance events like the founder effect contribute to a change in allele frequency.
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Summary
One misconception is that organisms evolve, in the Darwinian sense, during their lifetimes
Natural selection acts on individuals, but only populations evolve
Genetic variations in populations contribute to evolution
Microevolution is a change in allele frequencies in a population over generations
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The End